The invention relates to a method of concentration calibration of a stable isotope analyzer and to a stable isotope analyzer with means for concentration calibration.
In many fields of science (archeology, biology, geology, medicine, physiology, pharmacology) growing significance is attributed to the determination of isotope proportions. This applies primarily to the measurement of the 13CO.sub.2 /12CO.sub.2 ratio in the CO.sub.2 of the respiratory gas, which provides valuable information with respect to diagnostic queries. Stable isotope analyzers permit the performance of measurements of the isotope proportion, for example determination of the selective isotope proportion (*A.sub.x,B.sub.y) in a measurement gas (A.sub.x,B.sub.y) or the determination of the ratio at which the isotope proportions exists. When measuring the 13C/12C ratio, specifically in a respiratory gas stable isotope diagnostic, specification of the so-called Delta-value has found acceptance, where a measured relative value is related to a standard ratio.
With the aid of non-disperse infra-red spectroscopy, a method is known for selective determination of an isotope proportion of a measurement gas (EP-A-584 897). The appliance described by way of exemplary embodiment serves for measuring the 13CO.sub.2 /12CO.sub.2 ratio.
It comprises two ray paths or analyzers, one of which serves for measuring the 13CO.sub.2 -proportion and the other for measuring the 12CO.sub.2 -proportion.
When taking isotope proportion measurements, it has come to light that the result depends upon the concentration in which the measurement gas is present in another gas (for example CO.sub.2 in an inert gas) or a mixture of gas (for example CO.sub.2 in air). In order to obtain measuring results not subject to said dependence, it is necessary to undertake a concentration calibration, prior to taking measurements.
In the appliance according to EP-A-584 897, calibration of the analyzer is done with the aid of calibration cells, which can--inserted behind the measuring cells--be pivoted into the respective ray path. The calibration cells are filled with a mixture of an inert gas and the isotope-free measurement gas proportion belonging to the respective path of rays. CO.sub.2 -free air flows through the measuring cells during calibration.
In order to perform a concentration-calibration with an appliance of this type, a plurality of calibration cells must first be made available with different concentrations of isotope-free portions of the measurement gas in an inert gas. These must be pivoted, successively, into the ray paths and, after completion of taking the concentration measurement, must again be tilted out. From the plurality of the individual concentration measurements, which are respectively required for each ray path, it is possible to determine the dependence of the isotope portions measurements on the concentration in the inert gas.
The concentration calibration can also be performed without any mechanical pivoting of the appropriate calibration cells. In such case, however, one proceeds from the premise that the course of the signals is similar to the concentration for each of the two isotope proportions, and that there will be no error or only negligible errors in establishing the ratio.
It has be determined in actual practice that such an assumption is justified with respect to the accuracy of the measurements that need to be obtained. For this type of calibration, a series of gas test samples must be made available and successively measured, with varying and correspondingly closely graduated CO.sub.2 -concentrations and a constant Delta-value.
It is the object of the present invention to significantly simplify the concentration calibration of a stable isotope analyzer.
This object is solved according to the invention by the identifying characteristics of the Patent Claims.